Field of the Invention
The present invention relates to an image forming apparatus, and in particular to an image forming apparatus for forming a toner image on a printing medium.
Description of the Related Art
In a typical image forming apparatus, a voltage of −800 V is applied to a developing roller and a voltage of −1000 V is applied to a photosensitive drum, for example. The photosensitive drum is also irradiated with a beam by an optical scanner. The voltage of a part, irradiated with the beam, of the photosensitive drum becomes −100 V, for example. As a result, negatively charged toner does not move from the developing roller to a part, which is not irradiated with the beam, of the photosensitive drum, but moves from the developing roller to the part, irradiated with the beam, of the photosensitive drum. A toner image is developed in this manner.
In a typical image forming apparatus, adhesion of carriers to a photosensitive drum is prevented by an operation described below. FIG. 8 is a diagram illustrating a photosensitive drum 500, a charger 502, and a developing roller 504. FIG. 9 is a graph showing changes in voltage applied to the photosensitive drum 500 and voltage applied to the developing roller 504 while the voltages are raised. FIG. 10 is a graph showing a change in voltage at a part of the photosensitive drum 500 that the developing roller 504 faces and a change in voltage of the developing roller 504. Note that the change in voltage at the part of the photosensitive drum 500 that the developing roller 504 faces in FIG. 10 is obtained by delaying the change in the voltage applied to the photosensitive drum 500 of FIG. 9 by a time required for the photosensitive drum 500 to rotate from the charger 502 to the developing roller 504. In FIGS. 9 and 10, the vertical axis represents voltage and the horizontal axis represents time.
In the image forming apparatus, the absolute value of a voltage output by the charger 500 increases at a predetermined rate, and becomes constant after reaching 1000 V (−1000 V) at time T101. The absolute value of the voltage applied to the developing roller 504 also increases at a predetermined rate, and becomes constant after reaching 800 V (−800 V) at time T102. It is thus preferable that the voltage applied to the developing roller 504 reach −800 V at a timing when a part, to which a voltage of −1000 V is applied, of the photosensitive drum 500 reaches the developing roller 504.
Such a timing, however, varies. The voltage of the developing roller 504 may therefore reach −800 V before or after the part, to which a voltage of −1000 V is applied, of the photosensitive drum 500 reaches the developing roller 504. In particular, when the reaching of the voltage of the developing roller 504 is delayed, the difference between the voltage of the photosensitive drum 500 and the voltage of the developing roller 504 becomes large, which makes the electric field from the photosensitive drum 500 toward the developing roller 504 stronger. Positively charged carriers thus receive a greater force in a direction from the developing roller 504 toward the photosensitive drum 500. As a result, the force that the carriers receive by the electric field becomes greater than the force by which the carriers are attracted to the developing roller 504 caused by a magnetic force. The carriers thus move from the developing roller 504 to the photosensitive drum 500. Adhesion of carriers to the photosensitive drum 500 may cause damage to the image forming apparatus.
In view of the above, in the image forming apparatus, the voltage of the developing roller 504 is made to reach −800 V at time T102 that is a predetermined time T100 before time T103 at which the part, where the application of voltage is started, of the photosensitive drum 500 reaches the developing roller 504 as shown in FIG. 10. This prevents or reduces adhesion of carriers to the photosensitive drum 500.
A typical image forming apparatus, however, is disadvantageous in wasting toner. More specifically, as shown by a hatched region in FIG. 10, when the timing at which the voltage of the developing roller 504 reaches −800 V is made earlier, there is a period during which the voltage of the developing roller 504 is lower than that of the photosensitive drum 500. During this period, negatively charged toner moves from the developing roller 504 to the photosensitive drum 500.
Here, if the predetermined time T100 is shorter, wasteful consumption of toner will be reduced. The predetermined time T100, however, is a time determined in view of the variations in the rises of the voltage applied to the photosensitive drum 500 and the voltage applied to the developing roller 504, and the like. It is therefore difficult to shorten the predetermined time T100 to 0.
An image forming apparatus disclosed in JP 2012-113040 A, for example, is known as an invention relating to image forming apparatuses. In the image forming apparatus, the rise timing of a developing bias is controlled, so that wasteful consumption of developer will be reduced.